Frequency monitor



July 28,1942. H. o. PETERsN 2,291,558

FREQUENCY MONITOR Filed NOV. 14, 1940 i@ mr.

y ffm filter lNvENToR I BY ww/b ATTORNEY.

Patented July 28,

FaEQUENcY MoNrron Harold 0. Peterson, Riverhead, N. Y., assigner to Radio lCorporation of America. a comration I v of Delaware Application November 14, 1940, serial No. 365,584

' 1s claims. (ci. 25o- 39) The present invention relates to a means for Y detecting variations'in frequency of a transmitter from its assigned frequency.

An object of the present invention is the provision of a frequency monitor in -which the resonant circuits dovnot utilize lumped circuit elements.

Another object is the provision of a frequency monitor in which the resonators therein are substantially completely uncoupled one from the other.

A further object is the provision of a frequency monitor having an indicator which is direct reading in frequency departure fromv a predetermined value. v

Still a further object is the provision of a frequency monitor, as aforesaid, in which the scale of said .indicator is linearly calibrated.

The present invention includes as features of its construction'a pair of concentric line resonators tuned above and below a desiredfrequency Y and so adjusted that the resonance curves intersect at about 75% of the peak voltage values developed. The voltages developed in the reso- A nators are separately rectied and applied I through logarithmic instrument circuits-in opposing senses to a zero center voltmeter having a linear scale. thereon which may be calibrated in each Vdirection from the center directly in kilocycles departure from the desired frequency.

Further objects,v features and advantages of the present invention will be more fully understood from the following detailed description which is accompanied by a drawing in which Figure 1 shows the circuit diagram of an embodiment of the invention, Figure 2 shows a modication of a portion of Figure 1,`whi1e Figure 3 shows a further modification of Figure 2.

Referring, now, to Figure 1, there is shown therein a pair of concentric line resonators, one comprising an inner conductor I, and an outer shell 2, and the other an inner conductor 8 and an outer shell Il.l The inner conductorsA Iv and 3 are approximately a quarter wavelength ylong -at the desired resonant frequency. The inner conductors I and 3 of the resonators are grounded a larger ground plate or any other relative ar-` rangement desired may be used.

The radiated wave of the transmitter which is to be monitored is received on antenna 8 and55 positive voltage causes grid current\ to flow.

conducted through transmission -line 1 to the resonators I,2 and 3, 4.

The'transmission line 1 is split at 8 into two branches 8 and I0 and the inner conductorsof the branches are connected to the inner conductors of the resonators at points `II and l2. The lengths of the vbranches 9, I0 are such that the distance from point Il through point 8 to point I2 is approximately a quarter ,of the mean operating wavelength. The length of branches 8 and I 0 are thus chosen in order that there will be a minimum tendency for the branches to couple energy from one resonator tothe other.

One of the resonators, for example, the one composed of inner conductor I and outer shell 2 is tuned to a frequency above the desired or predetermined standard of frequency and the other composed of inner conductor 3 and outer shell 4 is tuned to a frequency below the desired or predetermined standard of frequency. The tuning of the two resonators is so adjusted, that is, the peak frequencies are so spaced that the vresonance curves ofthe two resonators intersect at a point equivalent to about '15% of the peak voltage values developed by the resonators.

Diode rectiers I5 and I6 are connected to the inner conductors of the resonators and serve to rectify the oscillatory-voltage in the resonators.

The point of connection ofthe rectifiers'to the inner conductors I, 3 may be varied if desired to change .the coupling of the load to the resonators. vAri increased coupling will broaden the tuning of the resonators and provide,v Within limits, for-a wider operating range. Load resistors I'l and I8 are by-passed by condensers I9 -and 20: for the radio frequency. The direct voltages developed across resistors .I1 and I8 are applied ythrough resistors 22 and 23 tothe grids of vacuui'ntubes 24 and 25. In the. load circuits of vacuumv tubes 24 and 25 are placed an indicatingfinstrument and load resistors 21 and 28. The v'potentiometer Sliconnecting load rewhen the frequency is correct. Diodes I5 and y I6 are contained within ak grounded shield chamsistors 21 and 28 and having its moving connection connectedto aysource of anode potential serves to aiford a means for adjusting the circuit to` obtain a zero reading on the instrument ber 2| in order to avoid stray eld pick-up which would lead to erroneous results. It will be noted that the direct voltages across the diode load resistors Il and I8 impress positive voltage on the grids of Atubes 24 and. 25. The impressed through resistors 22 and 23 but the voltage drop in resistors 22 and 23 is so adjusted that plate current of tubes 24 and 25 increases approximately as a logarithmic function of the voltages developed in resistors I 'land I8. In 5 practice, resistors 22 and 23 may be on the order of ten times or more greater than the value of resistors II and I8. With a logarithmic relationship between the direct current voltage, across resistors I1 and I8 and the plate currents in 10 tubes 24 and 25, the deflection ofthe indicating instrument 26 from its zero center is proportional to the departure of the transmitter frequency from the frequency to which the cross-over point of the monitor lcircuit is adjusted over a considerable range of input voltage. The instrument 26 may, therefore, be directly calibrated on a linear scale in terms of kilocycles, for instance.

The instrument 26 may be directly calibrated on l a linear scale because of the logarithmic relation-l ship described above. Ihe logarithmic transducer arrangement causes the deflection of the indicating instrument to be directly proportional to the deviation of the frequency of the wave applied to the resonators from a predetermined standard frequency. The following explanation makes this relationship apparent.A Consider the sloping portions of the resonance curves of resonators I and 3 as intersecting at a point which will be known as Eo. Then, at a frequency dif- 3() fering from the cross-over point the potential VEli' on resonator I is given by theA following equation:

where Af is the change in frequency. The poten- '35 tial E3 on resonator 3 is likewise given by the equation:

f Y E3=EoKAf From the two foregoing equations the4 following 40 proportion may be set up: Y

K E ljJfEAf E3- Then, since I Y s is small over the operating range of the device the foregoing proportion maybe written as the following approximation:

logarithm of the input potential it, therefore, follows that im is proportional to .e 2K EN '70 I have not indicated in the diagram the means for heating the cathodes of the tubes I5, I6, 24 and 25 since conventional cathode heating means are well understood in the art. Filamentary cathode tubes may be used if desired.' 7o

If it is desired to have the indication of" transmitter frequency departure appear at a location remote from the location of the monitoring device, the circuit of Figure'l may be modied to the right of dotted line, X, X, as shown in Figure 2. The voltage developed between the plates of tubes 24 and 25 is, in Figure 2, applied to the control electrode of the reactance tube 36. Reactance tube 36 is coupled to an oscillator circuit indicated generally by reference numeral 33 in such a way that variations in potential applied to the control electrode of tube 36 cause a 'change in the frequency generated by oscillator 38. The tone generated by oscillator 33 is applied through an output circuit to a monitor tone channel which carries the tone to a remote point where the indication is to be read. At this location, the received tone is applied to any desired type of frequency responsive indicating means.

'Ihe tone received at the remote point has a frequency which is dependent on whether or not the grids of tubes I8 and I9 receive equal voltages and the variation in tonefrecei'ved at the remote point is proportional to the departure of the signal received by antenna I' from the predetermined cross-over frequency for which resonators I, 2 and 3, 4 are adjusted,

In the further modification shown in Figure 3 I have shownthe use of a balanced-reactance tube circuit 4I, of the type shown and more fully described in Crosby application #311,074, filed December 27, 1939, to which reference may be made for a complete description of lits operation. In the present case it is believed only necessary to point out that the differentially varying voltages from the opposite ends of re- I sistors 21 and 28 are applied to the control electrodes of separate differentially acting reactance tubes. The varying/reactive effect is. in this modification, applied to a variable radio frequency oscillator indicatedgenerally by box 42 to vary the generated frequency an amount A above and below the assumed exemplary 101 kc. This variable frequency oscillator is then mixed with a fixed k. c. oscillation from generator 43 in a, mixer stage 44 whichmay include 'in its output a lter45 to pass only the resulting audio frequency note. This note is, if necessary, ampli- 0 fied by amplifier 46 and then applied to the monitor tone channel as indicated by character 40.

While I have shown and particularly described,

several modifications of my invention, it is to be distinctlyunderstood tha/t my invention is not limited thereto but may be varied within the scope of the invention.

I claim: y

1. A frequency monitor having a pair of resonant circuits tuned to frequencies above and below a predetermined standard, and means for applying a wave to be measured to said circuits, said means comprising a conductor connected between points on said resonant circuits and means for connecting a source of said wave to an intermediate point on said conductor, the length of said conductor from one circuit to the other being anodd multiple, including unity. of one quarter of the wavelength of said standard whereby said circuits are uncoupled one from the other.

2. A frequency monitor having a pair of resonators, each comprising an innerconductor and an outer shell surrounding said conductor. said resonators being tuned to frequencies above and below a predetermined standard frequency and means for lapplying ay wave to be measured to mission line connected between points on said resonators and means for connecting a source-of said wave to an intermediate point on said transmission llne, the lengthof said transmisison line from one resonator being an odd multiple, in-

assunse I said resonators, said means comprising a 'trans cluding unity, of one quarter of the wavelength f of said standard frequency whereby said resonators are substantially uncoupled one from theV other at said standard frequency.A

3. A frequency monitor having a pair of resonant circuits tuned to frequencies above and below a predetermined standard frequency, and means for applying a wave to be measured to said circuits, said means comprising a. conductor connected between points on said resonant cir-v cuits and the means for connecting a source of said wave to an intermediate point on .said conductor, the length of said,conductor from one circuit to the other being an' odd multiple, in-

cluding unity, of one quarter of the wavelength of said standard frequency whereby 4said circuits are uncoupled one from .the other, said circuits being so tuned that the point of intersection of the `resonance curves thereof occurs at a value equal to 75% of the peak value.

4. A frequency monitor` having a pair of resonators, each comprising a section of coaxial lines having an inner conductor and an outer s hell, the lengths of said inner conductors being so adjusted that said resonators are tuned to frequencies above and below a predetermined the resonance curves of saidresonator's occurring ata value equal to 75% of `the peak value, and means for applying a wave to be measured to said resonators, `said means comprising a transmissionline connected between points on each of Vsaid inner conductors andmeans for connecting a source 'of said wave to an intermediate wave to an intermediate v:point on said conductor, the length of said conductor from one circuit to the other being an odd multiple, including unit, of. one quarter of the wavelength of said standard frequency wherebysaid circuits are-uncoupled lone from the other, -means for developing direct potentials proportional to thealternating potentials developed by said wave in said circuits, means for developing currents |proportional to the logarithm of each of said directpotentials, and means for obtaining an indication proportional to the difference of' said cur-y rents.

'1. A `frequency monitor having a pair of resonators, each comprising a section of coaxial line means for connecting a source of'said wave to' l an intermediate point on said transmission lines,`

i standard frequency, the point of intersection of the length of said line from one inner conductor to the other being an'oddHmuItipIe,including unity. of one quarter of the wavelength'of said standard frequency whereby said resonators are uncoupled one from the other, means for de` cuits and means for connecting a source of said velop'mg direct potentials proportional tothe alternating .potentials developed. by said wave,in

l' said circuits, means for developing currents point on said transmission line, the length of saidz line from one inner conductor to the other being an odd multiple, including unity, of one quar-terl of the wavelength ofsaid standardfrequency whereby said resonators are uncoupled one from the other.

5. A frequency monitor having a pair of 'resonators, each comprising asection of coaxial line ,having an inner conductor and an outer shell, the lengths of said inner conductors being so adjusted that said resonators are tuned to frequencies above and below a predetermined standard frequency, the point of intersection of the resonance curves of said resonators occurring at a value equal to '15% of the peak value, and means for applyingv a wave to be measured to said resonators, said means comprising a transmission line connected between points on each of said inner conductors and means for connecting a source of said wave to an intermediate point on said transmission line, the length 'of said line from one inner conductor to the other being an odd multiple, including unity, of -one quarter of the wavelength of said standard frequency whereby said resonators are uncoupled one from the other, said resonators being arranged in an end to end relationship with a shielding plate therebetween.7

6. A frequency monitor having a'pair of resonant circuits tuned to frequencies above and below a predetermined standard frequency, and

means for applying a wave to be measured to said circuits, said means comprising a conductor proportional to the logarithm of` eachv ofsaid direct potentials and means for obtaining an inv--l .dication proportional tothe difference of said currents.

8. A frequency monitor having a pair of resonators, each comprising a section of coaxial lines having an inner conductor andan outer she1l,the lengthsof said inner conductors being such that said resonators are so tuned to'fre-A- quencies above and below a predetermined stand.

ard frequencythat the point of intersection of the resonance curves of said resonators oc'curs at a value' equal to 75% of the l.peak value, and means for applylng awave to be measured to said resonators, said means comprising a' transmission line connected to each of said inner con-- ductors and means for -connecting a source of connected between points on said resonant cirtween' said last mentioned said wave to an intermediate point on said transmission line, the length of said line from one inner conductor to the .other being an odd multiple, including unity, of one quarter of the wavelength of said standard frequency whereby said resonators are uncoupled one from the other,

said resonators being arranged in an end to end',

relationship with a shielding plate therebetween,

a pair. of rectiflers within a shield adjacent to said resonators, connections from each of said inner conductors to said rectiers, a load resistor connected to each of said rectiiiers, means for developing voltages proportional to the logarithm of the voltages developed across leach of said load resistors, means for combining said first mentioned voltages in anopposing relationship and means for indicating the differenceI therebe- .zero center voltmeter. f'

9. A frequency monitor having onant circuits tuned to freqencies abovel and below a predetermined standard frequency,

means comprising a vvto'r connected between points onl said resonant circuits, and means for connecting a source of said wave to an intermediate point on said concluding unity; of one quarter of the wavelength ascissa A `reaonatorsbeing tunedtoirequelicieaboveand I below a predetermined standard freq'encyrandY` means for wplyingawavetobe-measuredto said resonators,sai means comprisinga transjductor, the length of said conductor from onel circuitto the other being'anwodd multiple, in-

of said standard' frequency wherebyl salami-cuits are uncoupled on'e from the other, said circuits being so tuned that the point of intersection of 'the resonance curves thereof occurs at a -value equal to 75% of the peak value, means for developing direct potentials proportional to the al-A ternating potentials developed by said wave in said circuits, means for developing currents proportional to the logarithm ofeachvof said direct potentials, and means for obtaining an indication proportional to the difference of said curf rents.

10. A frequency responsive system including a pair of resonant circuits one tuned above, and

the other tuned below a predetermined frequency, a conductor connected between points on said circuits, and means for applying a waveto an intermediate point on said conductor, the length of said conductor from one circuit to the other being so related to the wavelength of said predetermined frequency thatl said conductor presents to each of said circuits a high impedance to said predetermined frequency whereby said circuits are uncoupled one from the other at said frequency. 4'

l1. A frequenct1 monitor having a pair of resonators each comprising an inner conductor and ,an outer shell surrounding said conductor, said mission line connec to'said resonators, and means'for connecting a sourceof said wa've to an intermediate point on said transmission line, the length of said line'irom one resonator to the other being'so related to the wavelength of said standard frequency that said transmission line presents a high impedance' to said standard l frequency whereby said resonators are uncou- 4 pled one from the other at said standard irequency.

12. A frequency responsive system having a pair of resonant circuits, one tuned above, and the other tuned below, a predetermined frequency, means for applying a wave to said circuits, means for Vdeveloping direct potentials proportional to the alternating` potentials de v veloped by said wave in said circuits, means for developing currents proportional to the logarithm of each of said direct potentials and means responsive to the difference of said currents.

13. A frequency monitor having a pair of resonant circuits tuned to frequenciesabove and below a predetermined standard, means for applying a wave to -be measured to said circuits, means -for developing direct potentials propertional to the alternating potentials developed by said wave'in said circuits, means for developing currents proportional to the logarithm of` each of said direct potentials and means for obtaining an indication proportional to 'the difierence of said currents.A

HAROLD O. PETERSON. 

